Microbial biofilms are formed when microorganisms adhere to a biotic or abiotic surface and produce extracellular macromolecules that facilitate adhesion to the surface and form a structural matrix that supports and protects the microorganisms. A biofilm is thus an accumulation of microorganisms such as bacteria embedded in an extracellular hydrated matrix primarily composed of exopolymers and other filamentous macromolecules, typically glycopeptides. Accordingly, a biofilm is generally described as a layer of bacteria (or other microorganisms), or as a plurality of layers and/or regions on a surface wherein bacteria are encased in a matrix of extracellular polymeric substances, or “EPS.” A substantial fraction of the biofilm is actually composed of this matrix; see, e.g., Donlan (2001) Emerging Infectious Diseases 7(2):277-281, Microorganisms in biofilms in many cases exhibit characteristics that are different from those seen with planktonic (freely suspended) microorganisms, particularly with respect to phenotypic traits like growth rate and resistance to antimicrobial treatment. It has been established that bacteria within biofilms can have up to a 1000-fold greater resistance to antibiotic agents than those grown under planktonic conditions, making eradication of a biofilm extremely difficult; see, e.g., Ceri et al. (1999) J. Clin. Microbiol. 37(6):1771-1776). One reason for this is the relative impenetrability of the biofilm—which can be both dense and thick—to antimicrobial agents. Another reason can be that the phenotype of sub-populations of cells in the biofilm changes so that the cells can better survive in the presence of antimicrobial agents; see Haagensen et al. (2007) J. Bacteriol. 189:28-37, and Folkesson and Haagensen et al. (2008) PLOSone, 3:e1891. Stability and resistance to dissolution are also key features of microbial biofilms; see Saville et al. (2011) J. Bacteriol. 193(13): 3257-64. An additional cause of antibiotic resistance may be that upregulation of efflux pumps can render biofilm cells able to transport unwanted antimicrobial agents out of cells in the biofilm; see Costa et al. (Oct. 27, 2011) BMC Microbiol. 11:241 and Nikaido et al. (2012) FEMS Microbiol. Rev. 36(2):340-63.
While biofilms can and do form on a variety of surfaces in a virtually unlimited number of contexts, biofilm formation in the medical arena is particularly concerning. As noted above, biofilm-related infections are extraordinarily tolerant to treatment with antimicrobial agents, and biofilm formation on medical implants is therefore extremely problematic. Microorganisms can attach to and develop biofilms on any type of medical implant, whether temporarily or permanently inserted or implanted in a patient's body, and can be a source of chronic bacterial infections. Chronic infections that are caused by biofilms on a medical implant (e.g., otitis media and osteomyelitis) often result in treatment failure and reoccurrence shortly after treatment. In 2005, biofilms accounted for about 65% of infections treated in the developed world. See Costerton et al. (1999) Science 284:1318-1322.
Medical devices are critical in modern-day medical practice. At the same time, they are major contributors to morbidity and mortality. The use of a medical device, particularly an implanted medical device or medical “implant,” is the greatest exogenous predictor of healthcare-associated infection; Manangan et al. (2002) Emerg. Infect. Dis. 8:233-236. Most infections that arise in the hospital setting, or “nosocomial” infections, occur primarily at four sites within the body: the urinary tract; the respiratory tract; the bloodstream; and surgical wound sites. According to Ryder et al. (2005) Topics in Advanced Practice Nursing eJournal 5(3), the following chronic diseases occurring in the nosocomial context have been established as caused by or at least associated with biofilms: cystic fibrosis; endocarditis; otitis media; prostatitis; osteomyelitis; chronic wounds; myeloidosis; tonsillitis; periodontitis; dental caries; necrotizing fasciitis; biliary tract infection; and Legionnaire's disease.
It has been found that 95% of nosocomial urinary tract infections are caused by an infected urinary catheter, 86% of nosocomial pneumonias are caused by an infected mechanical ventilator, and 87% of nosocomial bloodstream infections are associated with an infected intravascular device. See Ryder et al., supra, citing Richards et al. (1999) Crit. Care Med. 27:887-892. As will be explained infra, nosocomial bloodstream infections associated with an implanted catheter are the most life threatening of the aforementioned nosocomial infections and associated with the most significant medical costs.
The medical implants must be removed in order to remove the biofilm and then re-inserted into a patient's body. Examples of implantable medical devices on which biofilms may form include, without limitation:
Catheters, e.g., arterial catheters, central venous catheters, dialysis tubing, endotracheal tubes, enteral feeding tubes, gastrostomy tubes, hemodialysis catheters, nasogastric tubes, nephrostomy tubing, pulmonary artery catheters, tracheostomy tubes, umbilical catheters, and urinary catheters;
Implants, e.g., arteriovenous shunts, breast implants, cardiac and other monitors, cochlear implants, defibrillators, dental implants, maxillofacial implants, middle ear implants, neurostimulators, orthopedic devices, pacemaker and leads, penile implants, prosthetic devices, replacement joints, spinal implants, and voice prostheses; and
Other implanted devices such as artificial hearts, contact lenses, fracture fixation devices, infusion pumps, insulin pumps, intracranial pressure devices, intraocular lenses, intrauterine devices, joint prostheses, mechanical heart valves, ommaya reservoirs, suture materials, urinary stents, vascular assist devices, vascular grafts, vascular shunts, and vascular stents.
As indicated above, catheters are of particular interest because they are used in a host of medical applications and often involve critically ill and/or very young patients. Catheters are used not only in the administration of fluids and medication, but also in drainage of body fluids such as urine or abdominal fluids; angioplasty, angiography, and catheter ablation; administration of gases such as oxygen and volatile anesthetic agents; and hemodialysis. A central venous catheter (also referred to as a “central line” or “CVC”) is a widely used catheter that is placed in a large vein in the neck, chest, or groin and serves as a conduit for delivering medications, parenteral nutrition, and fluids. A CVC is commonly used in plasmapheresis, dialysis, and chemotherapy, and is also relied upon to obtain critically important measurements, such as central venous pressure (“CVP”).
Catheter-associated bloodstream infections (CABSIs; also referred to as catheter-related bloodstream infections, or CRBSIs) are a leading cause of morbidity and mortality in hospital settings. Each year 250,000 documented CABSIs occur in the United States, with an attributable mortality in the range of about 12% to 25% and an estimated cost to treat of $25,000 per episode ($6.2 billion annually, as of 2002). The intensive care environment accounts for 80,000 of these infections, with an attributable mortality as high as 35% and a cost to treat at $56,000 per episode. See Department of Health & Human Services, USA: Guidelines for the Prevention of Intravascular Catheter-Related Infections, 2011. Diagnosis is difficult and clinical suspicion of infection frequently leads to removal and replacement of indwelling catheters, resulting in significant healthcare costs and requiring that patients be subjected to additional procedures. The approaches that have been taken to counteract the widespread problem have not succeeded in either preventing biofilm formation or eliminating a biofilm that has formed without removal of the catheter from a patient's body.
While biofilm formation is generally problematic with implantable medical devices, it will be appreciated that the risk of infection is that much higher with catheters such as the CVC that remain in place for an extended time period. The most common bacteria found in CVC biofilms are Staphylococcus aureas, Staphylococcus epidermis sepsis, Candida albicans, Pseudomonas aeruginosa, Klebsiella pneumoniae, and Enterococcus faecalis. These bacteria may originate from patient's skin microflora, exogenous microflora from health care personnel, or contaminated infusions, and can migrate from the skin along the exterior surface or internally from the catheter hub or port.
It has been found that biofilm formation on CVCs is universal and that virtually all in-dwelling CVCs are colonized by microorganisms in a biofilm. Biofilms form not only on the outer surface of the catheter, but also on the inner lumen of the catheter, particularly with long-term catheterization; see Raad et al. (1998) Lancet 351:893-98.
The most prevalent approach to preventing CABSIs—hand washing and the use of aseptic techniques when handling the catheter—can be unreliable even in the highly controlled setting of a hospital. Other techniques such as ethanol lock therapy, or “ELT,” may degrade catheter materials and are not effective with respect to biofilms that are downstream from the inlet point. Catheters have been made with antibacterial coatings, including minocycline, chlorhexidine, and silver (see Aslam (2008), “Effect of Antibacterials on Biofilms,” Section of Infectious Diseases, Assoc. Prof Infect. Control Epidemiol. 5175:e9-e11), but the antibacterial efficacy of all of these coated catheters, wanes over time due to coating degradation; moreover, the coating method is not effective against nonbacterial organisms such as fungus, the coatings may selectively target only a particular type of bacteria, they can promote antibiotic resistance, and they are significantly more expensive than typical catheters. (Aslam, supra; Donlan, supra).
In the hospital setting, patients with indwelling catheters who have febrile illness and elevated inflammatory markers are suspected of having a CABSI. Blood cultures drawn from peripheral sites in these patients are compared with those drawn from the suspected catheter. If catheter cultures are positive, a line infection is suspected, particularly if peripheral cultures are negative. This method for verifying catheter infection is highly inaccurate, however, having a high false-positive rate because bacteria from other sources can also result in a positive test result. Thus, a catheter may be identified as infected when it actually is not. Currently, there is no highly specific, sensitive method for detecting catheter infection. Once a catheter is suspected of infection, first-line therapy is typically treatment with antibiotics. However, biofilm formation renders such therapy ineffective, as noted earlier, and antimicrobial agents can single out resistant organisms. In many cases, surgical removal of the catheter is necessary, resulting in increased healthcare costs, additional and sometimes unnecessary surgical procedures for patients, and reduction in potential venous access sites in patients who may be line-dependent for nutrition and pharmacotherapy.
The problem of infection is not limited to venous catheters, but also affects other types of catheters and medical devices as indicated above, such as urinary catheters, ventriculoperitoneal shunts, in-dwelling catheter-like prostheses (vascular conduits), dialysis tubing, endotracheal tubes, Foley catheters, and the like. Based on these considerations, a long-felt need is apparent for technology that can safely and effectively destroy a biofilm, i.e., kill microorganisms in the biofilm. Such a system would have widespread application in medicine, resulting in tremendous savings in healthcare costs, reduced morbidity and mortality, and assist in preventing further antibiotic resistance. It would also be optimal to provide a system that could not only kill microorganisms within a biofilm but also prevent biofilm formation and even sense the formation, presence, or growth of a biofilm. Ideally, such a system would be portable and easily controlled by a patient outside of a hospital setting. It would in addition be beneficial to be able to implement a system that meets the aforementioned requirements in the detection of a biofilm that has formed or is in the process of forming.